Embodiments of the present invention relates generally to reducing dislocations in a semiconductor structure. More particularly, the embodiments of the present invention are directed to methods of reducing dislocations in a germanium-containing layer formed on a semiconductor substrate having a dissimilar lattice constant.
Semiconductor devices often include multiple layers of conductive, insulating, and semiconductor layers. High mobility or high electron mobility is desirable in these devices. Germanium is one of the elements that has been shown to exhibit the high mobility characteristic. However, there are limited supplies of such element to satisfy the demand for high mobility substrates.
For many years, attempts have been made to grow high quality film on readily abundant substrate materials such as silicon. The high quality film can form a virtual substrate upon which semiconductor devices can be formed. Attempts have been made, for example, to grow germanium on silicon. These attempts have generally been unsuccessful because lattice mismatches between the host silicon substrate and the grown germanium comprising film, which has caused the resulting layer of germanium comprising film to be of low crystalline quality. For example, there is a difference in the lattice constant of silicon and the lattice constant of germanium. The two crystals thus have different lattice spacings and as such one atom cannot easily grow on top of another. Germanium is thus constrained by the underlying silicon substrate. When germanium is grown on silicon, the lattice spacing of germanium tends to try to match to that lattice spacing of silicon. When the germanium film is deposited to a sufficient thickness, only about 100 angstroms (or greater), the germanium layer relaxes causing dislocations (or defects) in the germanium layer.
Under many conventional methods of forming the germanium layer on a silicon substrate, a defect density of 109 per cm2 can be present in the germanium layer due to the dislocation formation. There are currently several techniques to reduce the dislocation (defects) in the grown germanium layer. In one example, (FIG. 1) the germanium layer is formed in a linear gradient with a steady increase of the percentage of germanium in the film from 0% to about 100%. Typically, the increase is graded at the rate of about 10% per every 1.0 μm of germanium layer. In this example, to grow a 100% germanium layer, the layer needs to be about 10.0 μm thick. Even with such a thick germanium layer, the dislocation or defects observed is still about 104 to 105 per cm2, which is substantially undesirable. Additionally, it has become apparent that it's impractical to grow a 10.0 μm thick germanium layer, for example, due to the long amount of time needed for the deposition of germanium.